Most thermoplastic (TP) polymers are not easily bonded using thermosetting (TS) structural adhesives such as epoxies, due to their inherently low reactivity, surface energies, and polarities. These difficulties can be resolved in a number of ways. The use of surface treatments to increase the wettability and polar nature of the thermoplastics will increase the potential for adhesive bonding to these materials. Techniques such as corona discharge, plasma treatments, acid etches, and oxidizing flame treatments have all been proposed as suitable surface preparation methods for thermoplastics which are not easily joined with thermosetting adhesives. While these approaches can be very effective, they have several disadvantages. Most notably, the treated surfaces do not maintain their characteristics for an infinite period. If the adhesive is not placed on the treated surface within a certain time period, the active surface's ability to promote strong bonding may be diminished or potentially eliminated due to contamination. Additionally, the equipment used in some of these procedures is quite expensive, and the maximum part size which can be treated may be restricted. Thus, much care must normally be taken when bonding thermoplastics with thermoset adhesives. A new technique with virtuality unlimited shelf life at room temperature is needed.
Joining of fully cured thermoset (TS) composite structural components has previously been limited to mechanical fasteners and traditional adhesives technology. Due to the high degree of crosslinking of cured thermosets, post-fabrication fusion bonding is not possible.
U.S. Pat. No. 5,264,059; U.S. Pat. No. 5,304,269 and U.S. Pat. No. 5,389,184 all issued to Jacaruso et al, and claiming priority to Ser. No. 628,245 filed Sep. 17, 1990, disclose a method of bonding thermoset composite structures to a pair of adhesive strips. The adhesive strips are comprised of a layer of semi-crystalline thermoplastic material. A heating element is used to bond them during fusion bonding.
We have found surprisingly that it is not necessary to fusion bond using a heating element. In addition, we found that it is also not necessary to use a layer of crystalline thermoplastic material.